The present invention relates in general to a retrofit fluorescent lamp dimming system for filament heated fluorescent lamps and more specifically to a fluorescent dimmer that is readily installable in a wall box.
Retrofit fluorescent lamp dimmers are capable of dimming the light output of fluorescent lamps connected in conventional ballasts by conditioning the AC power supplied to the ballast. In one exemplary prior art device, such as is shown in U.S. Pat. No, 4,350,935 to Spira et al., a portion or portions are removed from each half-cycle of the AC input waveform (by opening a series switch) resulting in periods of zero energy transfer to the lamps. The reduction in energy transfer results in a lowered light output from the lamps.
An improved fluorescent lamp dimming system of the prior art conditions the AC power supplied to the lighting system by chopping the input waveform in order to provide a variable duty cycle, low frequency input component which gives a variable light output, and to provide a high frequency component for maintaining filament heating at any amount of dimming without adding substantially to the light output. This improved system is described in U.S. patent application, Ser. No. 645,593 of Alley et al., filed Aug. 30, 1984, now U.S. Pat. No. 4,604,552, issued Aug. 5, 1986, which is a common assignment and which is hereby incorporated by reference.
In the above-mentioned dimming systems, a return circuit is connected across the ballast. The return circuit clamps the voltage across the ballast which could otherwise rise to extremely high levels during rapid switching of the current supplied to the inductive ballast load. In particular, the return circuit may be comprised of a switch connected across the ballast which closes only when power from the AC supply is switched off. In this way, the transients generated by the ballast during switching of the series switch are circulated through the return circuit, thus protecting the series switch from overvoltages and utilizing the transients to supply power to the lamps.
When retrofitting small lighting installations, such as a single room, for dimming, it would be desirable to have to access the system wiring only at the wall box, i.e. in the space provided for the room's on-off switch. However, only one wire is usually available at the wall box. Therefore, no return circuit can be conveniently used to clamp the ballast voltage and circulate the stored energy. It is also difficult to provide a supply of power for any control circuitry which might be used since only one terminal of the AC line is directly accessible.